The present invention relates to a method and an apparatus for assembling parts and more particularly to a method and an apparatus for fixing with adhesive a part and a part support for mounting the part via an intermediate member or members provided between the part and the part support.
Generally, to fix a part and a part support via a bracket-like intermediate member positioned between the part and the part support, it has been customary to fasten the part and part support and the intermediate member by using screws. Screws, however, are apt to displace the part relative to the part support due to a torque when they are driven, and thereby make it difficult to accurately position the part.
In light of the above, the part and part support and the intermediate member may be so configured as to mate with each other in a preselected positional relation. Although this approach enhances the positional accuracy of the individual structural element, it causes the positional accuracy of the resulting assembly to be unconditionally determined by the finishing accuracy of the individual element. It is therefore necessary to machine the individual structural element with high accuracy. While this kind of approach reduces the assembling cost, it increases the material and machining costs of the individual structural element. This is particularly true when the parts are plastic moldings apt to scatter in accuracy due to sinking and other causes.
To assemble the part and part support via the intermediate member without being effected by the finishing accuracy of the individual element while maintaining them in an accurate positional accuracy, it is desirable to connect the part, part support and intermediate member by using adhesive. This, however, brings about a problem that whether or not the part and part support are dislocated at the time of adhesion determines the positioning accuracy of the part relative to the part support after adhesion. It follows that the positional relation between the part and the part support at the time of adhesion has critical influence on the quality of the resulting product.
For example, assume that the above part is a print head included in a printer, a line sensor included in a scanner, or a solid imaging device included in a CCD (Charge Coupled Device) camera. Then, when any positional error occurs between the part and the part support, it displaces an image printed or read by the part and thereby deteriorates image quality.
Particularly, when the part is an ink jet head included in an ink jet printer, it occurs that the distance between the head surface of the head formed with nozzle holes and a recording medium is scattered or that the nozzle holes fail to accurately face a position where an image should be printed on the recording medium. As a result, ink drops ejected from the nozzle holes reach the recording medium outside of a preselected printing position, noticeably lowering the image quality. In the case of a color printer including heads respectively loaded with ink of different colors (usually yellow ink, magenta ink, cyan ink and black ink), any positional error between the heads makes the print positions of ink drops of different colors irregular. This brings the different colors forming a color image out of register or causes the color image to distort.
The prerequisite with the adhesive scheme is therefore that the part and the part support be accurately held, beforehand, in a preselected positional which will allow the part and part support to accurately face an assembly position at the time of adhesion. In this connection, in the case of the head of a color printer, the allowable error of the head adhered to the part support should be configured in the range of the order of microns.
As for the adhesive scheme, the positional relation between the part and the part support at the time of adhesion is a critical factor that determines the accuracy of mounting of the part to the part support, as stated earlier.
In light of the above, there has been proposed a part assembling apparatus of the type positioning the part support at a preselected position and holding it there, while holding the part in a position variable relative to the part support. By varying the position of the part, the apparatus adjusts a position in which the part should be mounted to the part support. An intermediate member is so positioned as to contact the part and part support. The apparatus applies photocuring adhesive to the interface between the part and the intermediate member and the interface between the intermediate member and the part support and the intermediate member for thereby fixing them together. This type of apparatus, however, has the following problem left unsolved.
If light for curing the adhesive applied to the interfaces is not uniformly distributed, a part of the adhesive is rapidly cured while the other part is slowly cured. As a result, the thickness of the adhesive layer differs from the part cured rapidly to the part cured slowly. Presumably, this is because the area of each interface over which the adhesive applied sequentially increases with the elapse of time due to, e.g., the surface tension of the adhesive. The irregular thickness f the adhesive effects the positional relation between the structural elements and thereby degrades the assembling accuracy of the structural elements.
The above problem will be solved if the light is uniformly radiated onto the adhesive. This, however, cannot be easily done because the gap available at the interface between the structural elements where the adhesive is applied is extremely small.
The intermediate member may be formed of resin transparent for light, as also proposed in the past. In this case, light is radiated onto the interfaces of the intermediate member via the intermediate members, so that the adhesive existing at the interfaces is cured at a substantially uniform rate. However, experiments showed that the light directly illuminating the adhesive via the intermediate member caused the composition of the transparent intermediate member to change and caused the member to color in muddy yellow little by little. The coloring of the intermediate member was particularly conspicuous when use was made of UV (Ultra Violet) rays as the light and UV curable adhesive as the adhesive.
Further, because the UV transmission of such colored intermediate member decreased, the UV rays could not fully cure the adhesive unless radiated for more than the expected period of time via the intermediate member, compared to the case of direct radiation. The decrease in the curing efficiency of the adhesive and therefore the extended radiation of the UV rays heated the intermediate member to such a degree that the member deformed.
In another conventional part assembling procedure, an intermediate member is positioned between the part and the part support. Adhesive is applied to a substantially vertical first interface and a substantially horizontal second interface between the part and part support and the intermediate member, thereby connecting the part and part support via the intermediate member. In this case, the adhesive is not always applied to each interface to a preselected thickness over a preselected area although it may be fed in a preselected amount. Specifically, adhesive use to mount the part usually has relatively high viscosity to as not to drop and is apt to protrude in the form of yolk when applied to the surface of the part due to the surface tension of the adhesive.
Assume that the structural members are assembled by the adhesive protruding from the surfaces of the members, as stated above. Then, it is likely that the area of the adhesive on each structural member is smaller than the expected adhering surface and causes the members to come off due to short adhesion strength. In addition, when the thickness of the adhesive differs from the first interface to the second interface, the structural members are displaced from each other when assembled. Moreover, the protuberance of the adhesive just after application is not constant, rendering the stress inside of the adhesive irregular during curing. Therefore, should the structural members be assembled without any processing following the application of the adhesive, the part would be inclined relative to the part support. In addition, it needs a long period of time for the adhesive protruding from the adhering surfaces to be cured, resulting in low productivity.
Furthermore, the liquid-like adhesive applied to the first interface is apt to drop due to its own weight or to turn round to the second interface. When the adhesive drops or turns round to any other position, the amount of the adhesive applied to the first interface and that of the adhesive applied to the second interface differ from the initial amount. As a result, the adhesive layers formed on the two interfaces are different in thickness from each other.
In the above condition, the positional relations between the part and the intermediate member and between the part support and the intermediate member are quite likely to differ from the time of position adjustment to the time of completion of the assembly. Errors in this kind of positional relations cannot be corrected by the position adjustment beforehand because the drop or the turn-round of the adhesive or an increase or a decrease in the amount of the adhesive ascribable thereto cannot be estimated. By contrast, errors ascribable to the contraction of the adhesive due to curing can be corrected by the position adjustment beforehand because the positional deviation of the individual member is proportional to the amount and area of application of the adhesive.